The production of magnetic recording materials requires the coating of a suitable backing with a multiplicity of magnetic domains. Typically, these domains are comprised of acicular particles whose magnetic moments are capable of responding to an externally applied magnetic field. The ability of these domains to respond in a desired way can be measured by several established parameters. Three important parameters of this type are saturation magnetization, coercivity, and squareness. These measure, respectively, the degree of saturation magnetization that can be obtained, the ability of the imposed magnetization to be maintained (rather than lost through the influence of adjacent domains) and the ability of the recorded medium to retain the signal in memory. All of these properties can be measured using standard methods, and provide a measure of the quality of recording which can be obtained on the medium.
A variety of materials have been used to provide the magnetic domains. Among these are chromium oxides, magnetic iron oxides, and metallic iron.
It has long been known that precipitation of iron(II) hydroxide in the presence of air results in a non-magnetic "alpha" form of ferric oxide which can, in turn, be converted to Fe.sub.3 O.sub.4 or magnetite through reduction by hydrogen or other suitable reducing agents. The resulting magnetite is, of course, magnetic but as it does not have desirable stability properties needed for preparing suitable recording media, it is then oxidized to the more stable "gamma" form of ferric oxide. Because magnetic or "gamma" Fe.sub.2 O.sub.3 is a cubic structure, it is difficult to prepare directly the desired acicular particles of this magnetic form of the oxide. The usual procedure is first to prepare the alpha (non-magnetic) form, (as above) which does easily form acicular particles, and then to convert this to the gamma (magnetic) form while retaining the physical shape of the original particles.
Various modifications of the foregoing scheme have been used to produce the magnetic particles, and a number of attempts have been made to optimize this preparation process. For example, U.S. Pat. No. 3,931,025 describes a magnetic iron oxide pigment containing both zinc and phosphate ions. The pigment prepared according to the disclosure of this reference is obtained by growing the alpha form of ferric oxide from a solution containing iron(II), zinc ion, and phosphate ion under conditions of oxidation. The presence of zinc and phosphate appeared to result in enhanced acicularity as ascertained by the length/width ratio of the particles, and to reduce the noise level associated with tapes prepared from them.
U.S. Pat. No. 4,321,302 discloses, as an incidental matter, the addition of zinc ion to the reaction mixture used in the original formation of the alpha ferric oxide; the improvement disclosed by this reference comprises the addition of silicate and phosphate ions to the original precipitating mixture. According to this reference, the presence of zinc ion appears to lower the coercivity of the resulting particles.
U.S. Pat. No. 3,912,646 also discloses the addition of modifying ions, including zinc ion, to the original solution used to obtain the alpha ferric oxide precipitate. The improvements which are the subject of U.S. Pat. No. 3,912,646, however, relate to the conditions of conversion of the alpha form to the magnetic gamma Fe.sub.2 O.sub.3.
In all of the foregoing cited references, zinc atoms or ions are retained in some form in the final composition. It is not clear from these references what, if any, influence the presence of zinc has on the magnetic properties of the particles obtained. In all cases, the zinc is included in the original alpha oxide precipitating solution.
Other modifications have been made by coating the gamma ferric oxide with metals or salts. U.S. Pat. No. 4,067,755 describes the provision of an alloy of cobalt and another element, which element may be zinc, as a coating on magnetic particles composed of either magnetite or gamma ferric oxide. The process comprises reduction of precipitated ions to the metallic form to obtain the alloy. Another coating which has proved helpful is the precipitation of both iron(II) oxide and cobalt(II) oxide onto the surface of a previously prepared acicular form of gamma Fe.sub.2 O.sub.3. The resulting particles retain their acicular character but contain a gamma Fe.sub.2 O.sub.3 core surrounded by an epitaxial layer composed of the oxides of iron and cobalt, the epitaxial layer having a crystal structure which appears to enhance the desirable magnetic properties of the particles. See, e.g., Hayama, F., et al, Ferrites: Proc. of Intl. Conf., Sept-Oct, 1980, Japan.